The present invention relates to a radio relay system comprising a plurality of radio relay devices for respectively relaying data by radio transmission, and particularly to a technology for improving the reliability of a radio relay system.
As a wireless or radio relay system comprising a plurality of radio relay devices for respectively relaying data by radio transmission, there has heretofore been known a radio relay system provided with a configuration shown in FIG. 27, for example.
In the illustrated radio relay system, radio relay stations 19 called xe2x80x9caccess pointsxe2x80x9d respectively relay data transmitted and received by radio terminals 3 and transmit the data to their corresponding radio terminals 3 set as destinations. Data transmission paths between the respective radio relay stations 19 are configured in tree form. Each of the radio terminals 3 selects one radio relay station 19 and always transmits and receives data via the selected radio relay station 19. The respective radio relay stations 19 respectively determine the directions to be relayed from destination addresses of data transmitted from their corresponding radio terminals 3 and other radio relay stations 19, and transmit data to their corresponding radio terminals 3 and other radio relay stations 19, based on the results of determination.
However, the radio relay system having the configuration shown in FIG. 27 has a problem in that since the data transmission paths are configured in tree form, a communication interrupt is easy to occur due to radio faults such as radio interference from other radio apparatuses, radio shielding developed due to a radio barrier, wave decay developed due to water vapor, etc., equipment failures, etc.
Namely, when a radio fault occurs between given two radio relay stations 19 as shown in FIG. 28 by way of example, each radio relay station 19 lying on the branch end side as viewed from a faulty point needs to search or find a new data-transmittable radio relay station 19 on the trunk side and make a change in data relay path. Further, even when a radio relay station 19 itself is disabled due to its failure as shown in FIG. 29, radio relay stations 19 lying on the branch end sides as viewed from the faulty radio relay station 19, and a radio terminal 3 directly connected to the faulty radio relay station 19 similarly need to search new data-transmittable radio relay stations 19 on the trunk side and make a change in data relay path.
Searching a new data-transmittable radio relay station 19 on the trunk side normally needs a time interval of from a few seconds to a few minutes. Therefore, the transmission of data between the radio terminal 3 lying on the branch end side as viewed from the faulty point or faulty radio relay station 19 and each radio terminal 3 which has performed the data transmission up to now through paths passing through the faulty point or faulty radio relay station 19, is made impossible, thus disenabling communications between these radio terminals 3.
There may also be cases where when the new data-transmittable radio relay station 19 on the trunk side cannot be found out as described above upon the occurrence of the radio fault or the failure in radio relay station 19, some radio terminals 3 would lapse into such an isolated state as to be unable to communicate with other remaining radio terminals 3 as shown in FIG. 29, for example.
In the radio relay system having such a tree-shaped configuration as shown in FIG. 27, as the radio relay station comes to or approaches each radio relay station 19 lying on the further trunk side, the number of radio terminals 3, which perform communications by way of such a radio relay station 19, increases and hence a load on its relaying process becomes great. Such concentration of loads on some radio relay stations 19 will degrade the communication performance of the entire radio relay system as a bottleneck.
Thus, the present invention aims to provide a radio relay system capable of increasing reliability with respect to a failure or bottleneck in radio transmission path. Further, the present invention aims to provide a radio relay system capable of avoiding the concentration of a load on a part alone.
There is provided a radio relay system according to the present invention, for achieving the above objects, wherein a plurality of radio relay devices are ring-connected by a plurality of radio transmission paths. When communication data received from an immediately preceding radio transmission path on a ring, which is formed by the ring connection, is not destined for each communication device held by a corresponding radio relay device, each of the radio relay devices is caused to relay the communication data to an immediately following radio transmission path on the ring. On the other hand, when the communication data received from the on-ring immediately preceding radio transmission path is destined for each communication device held by the corresponding radio relay device, the radio relay device is caused to transmit the communication data to each communication device held by the corresponding radio relay device. Further, the radio relay device is caused to transmit the communication data received from each communication device held by the corresponding radio relay device to the on-ring immediately following radio transmission path.
According to such a radio relay system, the load on the relaying process of each radio relay device increases as compared with the radio relay devices on the branch end sides in the radio relay system having the aforementioned conventional tree-like configuration. Since, however, the load on a relaying process of a specific radio relay device alone is not increased in particular as in the case of the radio relay devices on the trunk side, there is no possibility that this would result in a bottleneck, thereby degrading the communication performance of the entire radio relay system.
Incidentally, in the radio relay system according to the present invention, the plurality of radio relay devices may be ring-connected in double form through the use of the plurality of radio transmission paths in such a manner that the directions of data transmission are reversely turned relative to each other. Further, each of the radio relay devices may be caused to transmit the communication data received from the communication devices held by the corresponding radio relay device to immediately following radio transmission paths on two rings formed by the double ring connection with respect to the two rings. The radio relay device may be caused to transmit only one communication data of the same communication data which are received from the on-ring immediately preceding radio transmission paths with respect to the two rings and are destined for the communication devices held by the corresponding radio relay device, to each communication device held by the corresponding radio relay device.
If done so, then communications can be continued between arbitrary two radio relay devices excluding a fault-developed radio relay device through the use of either one of the two rings even when a failure occurs in part of each ring. It is thus possible to improve reliability.